The overall goal of this project is to describe the nature and function of the somatosensory system in the oral cavity using psychophysical methods. In previous years work has focused on thermal and tactile sensitivity and the interactions between temperature, touch and taste. In the present proposal attention is turned to the responsitivity of the oral mucosa to chemical irritation, which is believed to be mediated primarily by the trigeminal nerve. The project has two main thrusts: to investigate the basic perceptual characteristics of oral chemical irritation (the common chemical sense) and to elucidate its relationship to the somesthetic senses and taste. These objectives will be met by focusing research in three areas: (1) the phenomena of chemical sensitization and desensitization, (2) the interactions between chemical irritation and gustation. Chemical sensitization and desensitization, which we recently discovered can be rapidly induced on the tongue with relatively little discomfort, will be investigated in their own right as well as being used as psychophysical probes of the communality of the sensory pathways that mediate chemical, thermal and tactile stimuli. Studies of the interactions between chemical irritation and taste will include reinvestigations of cross-modal masking as well as tests of hypotheses about the possible roles of endogenous opioids and cognitive processes in the generation of apparent inhibitory interactions. Overall, the proposed studies will provide new information about the sensory and perceptual processes that underlie the sensitivity of the oral cavity to chemical irritants, and about the effects such irritants have on the sensitivity of the mouth to taste and the other cutaneous senses.